The invention is in the field of interference cancellation systems and methods, and in particular is an apparatus and method for cancelling at baseband frequencies interference components in a first composite signal using a second composite signal. The first composite signal has a desired carrier and an interfering carrier. The desired carrier has a constant envelope and is angle modulated with baseband .phi..sub.1 (t). The interfering carrier has any type of modulation. The second composite signal includes the interfering carrier.
An angle modulated carrier, whether in the context of telephony or video communications, is a carrier which is either phase modulated or frequency modulated by the modulating signal. In telephony, the modulating signal or baseband modulation may be a composite of a plurality of amplitude modulated frequency separated sub-carriers. In video communications, the baseband modulation is the video signal.
As is well known, R.F. carriers which are not sufficiently separated in frequency will interfere with each other causing distortion in the desired demodulation. Interference between R.F. carriers closely spaced in frequency occurs in many contexts. One such context which is of special concern to the assignee of the present invention is that of satellite communications. An example of a typical R.F. interference situation in satellite communications is shown in FIG. 1. The receiving antenna, typically having very high gain, high front-to-back ratio and directivity, is pointed at the desired signal source, satellite 1, so as to receive the desired carrier therefrom. An interfering carrier from an interfering signal source, satellite 2, is also received.
In certain cases, the desired and interfering carriers originate within the same satellite either from a cross-polarized signal or from a narrow beam satellite antenna pointed elsewhere, and the interfering carrier is sent with the desired carrier to the receiving antenna. Where dual polarization is used, a certain amount of R.F. signal leakage at the feed of the antenna is bound to occur. In this situation, the desired plus interfering carriers are received and amplified by the communications system and then fed to a bandpass filter having a passband tuned to the frequency of the desired carrier. No matter how steep the skirts of the filter, that portion of the interfering carrier which overlaps in frequency the desired carrier is passed by the filter and results after the demodulation process in an interference component superimposed on the desired baseband signal.
Various approaches have been devised to cancel or substantially attenuate the interfering carrier from the desired carrier while both carriers are in the R.F. domain. One such system is disclosed in U.S. Pat. No. 3,963,990 to Di Fonzo, issued June 15, 1976 and assigned to the assignee of the present invention. An article that describes the above invention and which includes a bibliography of relevant references on R.F. interference cancellation is: D. F. Di Fonzo, W. S. Trachtman, A. E. Williams, "Adaptive Polarization Control for Satellite Reuse Systems", Comsat Tech. Rev., Vol. 6, No. 2, 1976 at 253-283.
R.F. interference cancelling methods and apparatus are expensive to construct and maintain because of the complexity and necessary stability of the required circuitry. In order to overcome these deficiencies, the present invention performs the interference cancellation at baseband frequencies.